Within the prior art, various means have been developed to counter noise and/or vibration problems. These include passive treatments, passive Tuned Vibration Absorbers (TVAs), Adaptive TVAs (ATVAs), Active Noise Control (ANC), Active Structural Control (ASC), and Active Isolation Control (AIC) all of which will be briefly described herein. Passive treatments, such as sound-deadening blankets, are generally effective in attenuating higher-frequency noise, but are generally ineffective at attenuating low-frequency noise, for example, low-frequency engine tones. Notably, passive blankets must be relatively massive to reduce low-frequency noise transmission into a vehicle's cabin. Therefore, other mechanisms are generally employed for low-frequency vibration/noise suppression.
Passive Tuned Vibrations Absorbers (TVA's) are known devices which find utility in absorbing low-frequency vibration to provide local vibration reduction at their attachment point. TVAs may also be effective at cancelling low-frequency noise within a vehicle's cabin which is radiating from the surrounding structure. Although, TVAs are generally well adapted for attenuating low-frequency noise, they are generally somewhat limited in range and effectiveness. As shown in Prior Art FIG. 1, passive TVAs include a suspended tuning mass 32 which is tuned (along with a stiffness of spring 30), such that the device exhibits a resonant natural frequency (fn) which generally cancels or absorbs vibration of the vibrating structure 22 at the point of attachment thereto. The afore-mentioned disadvantage of passive TVAs is that they are only effective at a particular frequency (fn) or within a very narrow frequency range thereabouts. Therefore, TVAs may be ineffective if the disturbance frequency is changed, such that the TVA is not excited at its resonant frequency (fn). Moreover, passive TVAs may be unable to generate proper magnitude or phasing of forces needed for effective vibration suppression and/or control. In aircraft, passive TVAs may be attached to the interior stiffening rings or stringers of the fuselage or to the yoke. U.S. Pat. No. 3,490,556 to Bennett, Jr. et al. entitled: "Aircraft Noise Reduction System With Tuned Vibration Absorbers" describes a passive vibration dampening device for attachment on the yoke of an aircraft for absorbing vibration at the N1 and N2 rotational frequencies.
When a wider range of vibration cancellation is required, various adaptive TVAs may be employed. For example, U.S. Pat. No. 3,487,888 to Adams et al. entitled "Cabin Engine Sound Suppresser" teaches an adaptive TVA where the resonant frequency (fn) can be adaptively adjusted by changing the "length" of a beam, or the rigidity of a resilient cushioning material. Although, the range of vibration attenuation may be increased with adaptive TVAs, they still may be somewhat ineffective for certain applications, in that their range of adjustment may not be large enough, or they may not be able to generate large enough dynamic forces to dramatically reduce acoustic noise or vibration experienced within a vehicle's cabin, albeit, under certain circumstances they may be quite effective.
In some applications where a higher level of noise and/or vibration attenuation is desired, Active Isolation Control (AIC) systems may be used for controlling noise/vibration within the vehicle. AIC systems include "active mountings" which are attached between the engine (disturbance source) and its attachment structure (frame, pylon, etc.). Active mountings include an actively driven element therein, which provides the active control forces for isolating vibration and preventing its transmission from the engine into the vehicle's structure. The resultant effect is a reduction of annoying interior acoustic noise, as well as a reduction in vibration, in most cases. Known AIC systems include the feedforward type, in which reference signals from reference sensors are used to provide a signal indicative of the engine vibration(s) to the control process. Likewise, error sensors provide error signals indicative of the residual noise/vibration. These reference and error signals are processed by the digital controller to generate output signals of the appropriate phase and magnitude (anti-vibration) to drive an output active mounting to reduce vibration transmission from the engine to the structure, and resultantly control the interior acoustic noise and structural vibration.
U.S. Pat. No. 5,551,650 entitled "Active Mounts For Aircraft Engines" describes one such AIC system. Furthermore, commonly assigned U.S. Pat. No. 5,174,552 to Hodgson et al. entitled "Fluid Mount With Active Vibration Control" describes one type of active fluid mounting. Notably, it should be understood, that in some applications there may be insufficient space envelope to house the active elements within the active mounting. Further, there may be alternate vibration paths into the structure, or the appropriate actuation directions required for good vibration attenuation may be difficult to achieve within the space constraints of the active mount. Therefore, under these circumstances, other types of active control may be implemented, such as Active Noise Control (ANC) or Active Structural Control (ASC).
Active Noise Control (ANC) systems are also well known. ANC systems include a plurality of acoustic output transducers, such as loudspeakers, strategically located within the vehicle's cabin/passenger compartment. These loudspeakers are driven responsive to input signals from input sensors representative of the disturbance and error signals from error sensors disbursed within the vehicle's cabin. Input signals may be derived from engine tachometers, accelerometers, or the like. The output signals to the loudspeakers are generally adaptively controlled via a digital controller according to a known feedforward-type adaptive control algorithms, such as the Filtered-x Least Mean Square (LMS) algorithm, or the like. Copending U.S. patent application Ser. No. 08/553,227 to Billoud entitled "Active Noise Control System For Closed Spaces Such As Aircraft Cabins" describes one such ANC system. Further discussions of ANC systems may be found in U.S. Pat. No. 5,526,292 to Hodgson et al. entitled "Broadband Noise And Vibration Reduction." ANC systems have the disadvantage that they do not address any mechanical vibration problem that may exist, and may be difficult to retrofit in certain vehicles. Furthermore, as the frequency of the noise increases, larger numbers of error sensors and speakers are required to achieve sufficient global noise attenuation.
Certain ASC systems utilizing AVAs, known in the prior art, may solve this problem of needing a large number of error sensors by attacking the vibrational modes of the vehicle's structure directly. For example, by attaching a vibrating device, such as an inertial shakers or AVAs to the interior surface of the fuselage, as described in U.S. Pat. No. 4,715,559 to Fuller, global attenuation can be achieved with a minimal number of error sensors. However, the modifications necessary to retrofit AVAs in this manner may be prohibitive, as the interior trim may have to be removed and structural modifications made have to be made to the stringers or stiffening-ring frames. Furthermore, for control of higher order tones, a large number of AVAs may be needed, thereby requiring large power requirements for each AVA and associated amplifier. Therefore, prior art ASC systems are necessarily difficult to retrofit and may require the use of many inertial shakers to effectuate control of higher-order tones. U.S. Pat. No. 5,310,137 to Yoerkie, Jr. et al. describes the use of AVAs to cancel high-frequency vibrations of a helicopter transmission. Notably, Yoerkie, Jr. et al. is a feedback-type system.
As described in Prior Art FIG. 2, Active Vibration Absorbers (AVAs) comprise a tuning mass 32, a housing 28, a spring 30 flexibly supporting the tuning mass 32, and a force actuator 40 (coil and magnet assembly or the like) for actively driving the tuning mass 32 along its acting axis A--A. The stiffness of spring 30 and mass of tuning mass 32 may be tuned such that the AVA is more easily driven at its predominant frequency. Prior Art FIG. 3 describes a Multiple-Degree-of-Freedom Active Vibration Absorber (MDOF AVA). MDOF AVAs include an extra flexible member 26. The mass of housing 28 and stiffness of additional flexible member 26 are tuned to provide a second resonant frequency. Further descriptions of AVAs and MDOF AVAs can be found in Copending U.S. application Ser. No. 08/322,123 entitled "Active Tuned Vibration Absorber", copending PCT application PCT/US95/13610 (WO 96/12121) entitled "Active Systems and Devices Including Active Vibration Absorbers (AVAs)", U.S. Ser. No. 08/698,544 entitled "Active Noise and Vibration Control System", U.S. Ser. No. 08/693,742 entitled "Active Structural Control System and Method Including Active Vibration Absorbers (AVAs), and U.S. Ser. No. 08/730,773 entitled "Hybrid Active-Passive Noise and Vibration Control System for Aircraft." FIG. 4 illustrates one prior art preferred implementation for achieving active forces in multiple directions. The AVAs (which could also be MDOF AVAs) are attached to rigid bracket 38 which attaches to structure 22 via fastener shown. The inertial shakers/AVAs 25, 25' shown are actively driven along their acting axes at the appropriate frequency, amplitude, and phase to appropriately control noise and/or vibration.
The individual AVAs described above suffer from the problems that they are either mass inefficient, incapable of multiple direction actuation, or require large amounts of electrical power. Therefore, there is a long felt and recognized need for an AVA assembly which provides multi-directional active vibrational forces to effectively control vibration within the structure, which is efficient, and which minimizes mass and power requirements for generating the needed cancellation forces.